Impact of prion replication within neurons on their sensitivity to pro-inflammatory cytokines (TNFa) – Prions&SensiTNF

Our studies exploit the properties of a neuronal stem cell endowed with the capacity to differentiate into serotonergic neurons as well as primary cultures of neurons arising from the mouse brain. These distinct cell paradigms all support prion infection. The relevance of our in vitro data is tested in vivo with mice inoculated by pathogenic prions. Concerning « Alzheimer » studies, we exploit transgenic mice, which display Alzheimer’s symptoms as soon as 8 month of age.

Our work establishes that an enzyme with a protease activity (TACE) vanishes from the cell surface of prion-infected or Alzheimer’s neurons. In fact, this protease is found inside neurons where it is not active, thus leading to the accumulation of proteins pathogenic for neurons. We demonstrate that internalization of TACE protease depends on another enzyme, PDK1. Antagonizing PDK1 activity is sufficient to protect prion-infected and Alzheimer mouse models from the disease.

In view of the absence of efficient therapies to face up prion disorders (Creutzfeld-Jakob) and Alzheimer's disease, our overall data posit PDK1 as a potential therapeutic target to combat prion and Alzheimer’s diseases. Nevertheless, the design of new PDK1 inhibitors non-toxic for the body is a priority. From a fundamental point of view, our future studies aim at understanding why PDK1 is deregulated in prion- and Alzheimer’s-diseased neurons.

A paper describing the critical role of PDK1 in prion and Alzheimer's diseases and PDK1 as a potential therapeutic target for these neurological disorders has been submitted for publication in a top-ranked journal.
« PDK1-dependent switch-off of TACE shedding activity in prion and Alzheimer’s diseases »

Transmissible Spongiform Encephalopathies (TSEs), commonly named prion diseases, are a group of neurodegenerative disorders that includes scrapie and bovine spongiform encephalopathy in animals and Creutzfeldt-Jakob disease in humans. Prion diseases are characterized by a widespread neuronal dysfunction and loss, spongiosis, and accumulation of PrPSC, the pathological isoform of a host-encoded prion protein, PrPC. To date, TSEs constitute a class of irreversible and fatal neurodegenerative diseases for which no effective medecine are available.

The autocatalytic conversion of PrPC into PrPSC lies at the root of prion diseases. The central role played by PrPC in the development of prion diseases was first illustrated by the observation that disruption of the PrP gene in mice confers resistance to prion disease and to the propagation of infectious prions, while PrP-overexpressing mice exhibit reduced incubation periods as compared to wild-type mice. Mallucci et al. further showed that switching off PrPC neuronal expression in infected mice blocks TSE pathogenesis, although abundant prion replication still occurs in glial cells. These data outline that prions require neuronal PrPC to exert their toxicity.

Subversion of membrane GPI-anchored PrPC-associated function(s) in neurons as a result of the conversion of PrPC into PrPSC is assumed to account for prion-associated toxicity. Whether PrPSC triggers a loss of PrPC physiological function (loss-of-function hypothesis) or promotes a gain of toxic activity (gain-of-function hypothesis), or both, is an ongoing debate in the TSE field. Elucidating PrPC roles in neurons should help to answer that question. A stress-protective activity was assigned to PrPC. The neuroprotective role of PrPC has to be linked to its engagement in cell signaling events. The interaction of PrPC with the stress inducible protein (STI-1) induces neuroprotective signals that rescue cells from induced apoptosis. Our previous studies substantiate the coupling of PrPC to signaling effectors involved in cell survival, redox equilibrium and homeostasis such as MAP kinases ERK1/2, NADPH oxidase, CREB transcription factor and metalloproteinases.

Despite these overall advances, the sequence of cellular and molecular events that lead to neuronal cell demise in TSEs still remains largely unknown. At present, one envisions that neuronal cell death results from several parallel, interacting or subsequent pathways that involve protein processing and proteasome dysfunction, oxidative stress, autophagy and apoptosis. Beyond the direct toxicity exerted by PrPSC on neurons, inflammation of the central nervous system represents an underlying component of prion diseases that very likely contributes to PrPSC-induced neurodegeneration. PrPSC aggregates accumulate within microglial cells and astrocytes and stimulate the release in the neuronal microenvironment of a set of neurotoxic factors, including chemokines, inflammatory cytokines, reactive oxygen species, that all might contribute to neuronal cell death. However, whether PrPSC replication and accumulation within neurons exacerbate the toxic action of inflammatory cytokines remains largely unknown.

Our hypothesis is that PrPC normally exerts in neurons a cytoprotective role towards pro-inflammatory cytokines. The neuronal protective function would depend on the signaling activity associated to PrPC. In prion-infected neurons, PrPC conversion into PrPSC would cancel the PrPC protective function, leading to an increased vulnerability of neurons exposed to high levels of inflammatory cytokines secreted by microglia during prion infection. This proposal aims at investigating: 1) whether and how PrPC exerts a protective role against TNFa, 2) whether PrPSC replication within neurons triggers a loss of the PrPC protective effect against TNF-a, and 3) whether pharmacological inhibition of critical signaling effectors desensitizes infected cells to TNFa toxicity.